6” f/4 Newtonian
My 6 inch f/4 Newtonian has a carbon fiber tube and a 2.6 inch secondary. As I said earlier, the mirror, which was f/6, came from my old Criterion Dynascope. Here is the mirror before I started to regrind for a faster focal ratio. I think the figure that I put on the mirror turned out okay. According to Foucault, it is ⅛ or better – tough to really know as it is somewhat challenging for me to read the zones on a fast mirror. Ronchi shows a very good match to that generated by Mel Bartel’s Ronchi program (found here). My highest power eyepiece is a 5 mm. I really needed a barlow or stronger eyepiece for a proper star test. However, with the 5 mm, the mirror seemed good. I made a 30 inch carbon fiber tube. The inside diameter is 8 inches and the wall thickness about 0.2 inches. I used 6 wraps of carbon fiber (6K 2×2 Twill) and covered that with a 6K carbon fiber sleeve (I thought the sleeve would give a bit better finished appearance to the tube). I then covered the tube with a heat shrink plastic tube and proceeded to use a heat gun to compress the carbon fiber layers. With 6 layers (7 counting the sleeve), the thickness should have been about 0.1 inch but considering I couldn’t wrap the carbon fiber that tight and the time it took to epoxy and roll, I ended up with a tube thickness of about 0.2 inches. It is still really strong and will not flex. Like my 8 inch and 12.5 inch scopes, I made tube rings from laminated maple strips. They ended up about 1 inch thick and 1.25 inches wide. Here is a picture of the 6 inch f4 on my Mesu mount in warm Florida.
12.5” f/3.6 Newtonian
Picture of my 12.5 inch scope on my concrete pier in Glenwood, MN.
UPDATE: In the section below I said I use an ST-8300C and and Orion OAG. I have since switched to a Player One Poseidon-C Pro and Player One’s Filter Drawer MAX and FHD-OAG MAX. I use this combination with my 6 inch also. It’s been an awesome camera.
I completed my 12.5 inch f/3.6 Newtonian in October 2017, approximately 2 years after conception. The mirror blank (pyrex) came from Newport Glass. I was actually shooting for a f/3.8 but ended up with a f/3.6. I ground and polished the mirror and then attempted to figure it. I’ve never figured anything this fast and had all sorts of problems trying to work the figure. I ended up contacting Steve Swayze to do the figuring. Unfortunately, he passed away in 2021 after a battle with cancer. He did a wonderful job and it was stress-free on my part! Because the scope is fast, I needed a coma corrector and decided to use the Paracorr Type-2 from Televue. On a number of pages I refer to this telescope as f/3.6. However, the Paracorr 2 brings in a multiplication factor of 1.15 so the real focal ratio is more like 4.15. My secondary and spider are from Astrosystems. I purchased a used SBIG color camera (ST-8300C) from Chuck Pavlick (see his astrophotos here). I was going to use an off-axis guider but decided to try using a guide scope first. I used an Orion 60 mm guide scope with a StarShoot Autoguider Pro Mono camera. Unfortunately, Orion is no longer in business. I also have a Focuser Boss II from Starlight Instruments for electronic/computer focusing. As a focusing aid, I made a Bahtinov mask using a black foam core board and the mask generator from Astrojargon. I have since gone back to an OAG (Orion Thin Off-Axis Guider) and changed to an ASI290mm mini as guide camera. I use this for both the 12.5” and the 6” scopes. The interior of the tube is lined with self-adhesive black acrylic flocking material (www.scopestuff.com). With everything loaded on my telescope, it weighs about 60 lbs. Fans on mirror cell are from Digikey and are Sunon MagLev fans (MB50101V2-00U-A99). I have dew controllers for my secondary and Telrad from Kendrick Astro Instruments. I’ve used this telescope at locations where power is supplied but I do have a battery-power supply for remote areas. I used to use a Pyramid PS9KX power supply (13.8 v DC output) for powering my scope and accessories. However, that power supply crapped out so I bought a Powerwerx 30 amp power supply. Below is a picture of the scope at the 2017 WSP.
Unfortunately, I had a lot of trouble with flexture with this scope so I limitied its use. Thinking the primary mirror cell was the problem, I added a thin piece of aluminum to the back of the cell thinking the cell itself was flexing. I also added stronger springs for the tilt adjustment. But … this did not help. Then I came upon an article by Mike Lockwood on mirror support – both primary and secondary mirror support (article here). Near the bottom of the article he discusses the secondary mirror. I felt like an idiot because I was doing exactly what he said not to do – my secondary mirror holder was too far from the spider so all it had for support was the main bolt. I moved it in eliminating all distance between the secondary holder and spider. Now the back of the secondary mirror holder is resing on the 4 vanes of the spider and the flexture I was noticing is gone. Such a simple solution but I was just not seeing it. I had to slightly move the primary mirror cell to account for the movement in the secondary, but it was worth it – obviously!
Picture of my 6 inch scope on my concrete pier in Glenwood, MN. Picture was taken in November 2021. My electronics (computer and power supply) are in the cooler. The heat put off by the computer and power supply was enough to keep the computer from shutting down as the temperatures were getting to well below -10° F.
